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Evaluation of the Oxygen-Induced Zircaloy Embrittlement in ICARE/CATHARE

ALIAS CZ. Evaluation of the Oxygen-Induced Zircaloy Embrittlement in ICARE/CATHARE. Ladislav Belovsky ALIAS CZ s.r.o., Czech republic belovsky@telecom.cz Presented at the 11 th International QUENCH Workshop , October 25-27, 2005 , Forschungszentrum Karlsruhe, Germany.

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Evaluation of the Oxygen-Induced Zircaloy Embrittlement in ICARE/CATHARE

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  1. ALIAS CZ Evaluation of the Oxygen-Induced Zircaloy Embrittlement in ICARE/CATHARE Ladislav Belovsky ALIAS CZ s.r.o., Czech republic belovsky@telecom.cz Presented at the 11thInternational QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany

  2. Motivation for Development of a New Model (ZROB) • ICARE/CATHARE: … applicable also for LOCA and beyond DBA analyses • Acceptance criteria for ECCS for LWRs ( 10CFR50.46 ):Evaluation of post-quench cladding embrittlement (17% ECR by B-J since 1973). • The 17% ECR criterion currently under revision by USNRC: • High burnup (hydrogen, pre-oxide). • New Zr-based alloys • Exp. research indicates that embrittlement is a combined function of : • Oxygen content & distribution metal (beta phase) • Hydrogen content (& distribution ?) in metal • Modeling of Zircaloy embrittlement in ICARE/CATHARE in two steps: • 1. step: Oxygen-induced embrittlement (O-diffusion in beta phase) • 2. step: Impact of hydrogen onto embrittlement (O-solubility & diffusion, hydrides, …)

  3. Modeling Features & Assumptions • ZROB receives beta layer boundaries from oxidation module (ZROX or UZRO) • ZROB calculates 1D oxygen diffusion in beta layer >970 °C (oxygen-free ZR in ZROX): • Oxidizing surface: - Beta layer (ZR) always covered with O-stabilized alpha layer (ZRO). - Boundary concentration at ZR/ZRO: Zircaloy-Oxygen phase diagram • Non-oxidizing surface: Zero oxygen flux. • Uniform meshing, Cylindrical coordinates, Implicit finite-difference method, Gauss elimination. • Initial condition: Constant concentration profile (as-receivedmaterial). • ZROB deduces from the oxygen concentration profile in the beta layer : • Thickness of beta layer with less than specified O-concentration ( …, 0.6, 0.7, … wt% O ). • Fractional saturation of beta layer. •  embrittled Zircaloy components after quenching (Chung-Kassner 1 and/or Pawel 2 criterion).

  4. Diffusion Equation for Oxygen in ZR Layer • Oxygen mass balance in ith segment: • Oxygen fluxes at segment boundaries:Ji = Di·ΔC/ΔR Example of two-sided oxidation C1 CN i-th segment (regular) Ji+1 Ji JN+1 J1 C2 CN-1 Ci-1 Ci+1 Ci outer segment inner segment rN+1 rN rN-1 ri+1 ri ri-1 r2 r1 C RN RN-1 Ri+1 Ri Ri-1 R2 R1 r thickness of ZR layer

  5. 970 820 °C Oxygen diffusion coefficient in ZR layer > 970 °C (-Zr) : D = 2.63·10-6 exp(-28200/(1.987·T)) J. Nucl. Mat. 68 (1977) < 820 °C (-Zr) : D = 1.32·10-4 exp(-48200/(1.987·T)) J. Nucl. Mat. 67 (1977)

  6. Oxygen solubility SO in ZR layer at ZR/ZRO interface • >1007 °C : SO = exp(5.02 – 8220 / T[K]) [wt%] As-received Zircaloy (Chung-Kassner3) • 970-1007 °C : SO = 5.246·10-3·(T[K]-1233) • < 970 °C : SO = 0 T [ C ] Phase diagram Zry-O Oxygen concentration [ at% ] outer surface ZrO2 ZRO -Zr SO ZR -Zr inner clad surface

  7. Input & Output Data • Input: • MACR xxxx User name of the oxidizing Zircaloy macro-component (eg. CLAD1). • CINI Initial O-concentration in as-received Zircaloy: 0.1 wt% ( 0 - 1.5 ) • COXX User defined critical O-concentration: 0.55 wt% ( 0 - 2 ) • DTMX Max. length of internal sub-time step within global Δt: 0.5 s ( 0.001 - 10 ) • NMAX Max. number of concentration points in ZR: 15 ( 4 - 100 ) • Output: • Fractional saturation of beta layer FBS = CAV / SO • CAV : Average concentration of oxygen in ZR layer • SO : Boundary concentration of oxygen in ZR (oxygen solubility) • Thickness THICXX within ZR with max. COXX [wt%] oxygen(another six variables THIC04 to THIC09 are automatically calculated for 0.4 to 0.9 wt%) • If embrittlement criterion fulfilled < 400 K (Chung-Kassner 1 or Pawel 2), component state  DISLOCAT.

  8. Results: Numerical Against Analytical Solution • Non-moving boundary diffusion problem in a slab, thickness l = 0.7 mm • Outer surface: oxidizing, boundary concentration from Chung-Kassner 3 correlation • Inner surface: zero oxygen flux • Initial O-conc.: 0.1 wt% (1000 wt ppm) • Constant temperature 1000 °C, 1400 °C • Analytical solution (Carslaw & Jaeger 4): • Oxygen concentration C(x, t) after t seconds at distance x from the surface: • Numerical solution by ZROB : • Clad diameter 9 m ( slab) • Default input data • Comparison: Good agreement (see next figures)

  9. 18 1.8 Analytical 16 1.6 ZROB ] 14 1.4 60 s 3 Rel. difference [%] 12 1.2 10 1.0 1140 s 8 0.8 400 s Rel. difference (A - ZROB) / A [%] Oxygen concentration [kg/m 60 s 6 0.6 4 0.4 400 s 1140 s 2 0.2 0 0.0 -2 -0.2 -4 -0.4 0 100 200 300 400 500 600 700 Distance from the outer cladding surface [micron] Results: Numerical Against Analytical Solution Cont’d Temperature 1000 °C

  10. 100 2.0 Analytical 90 1.8 ZROB 80 1.6 ] Rel. difference [%] 3 40 s 70 1.4 480 s 60 1.2 50 1.0 Rel. difference (A - ZROB) / A [%] 0.8 40 Oxygen concentration [kg/m 40 s 30 0.6 20 0.4 10 0.2 0 0.0 480 s -10 -0.2 -20 -0.4 0 100 200 300 400 500 600 700 Distance from the outer cladding surface [micron] Results: Numerical Against Analytical Solution Cont’d Temperature 1400 °C

  11. 8 6 40 s 7 5 40 s 480 s 6 480 s 4 5 3 4 Max. relative difference [%] Max. relative difference [%] 3 2 2 1 1 0 0 -1 -1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 10 20 30 40 50 60 70 80 90 100 Internal sub-time step DTMX [ s ] Number of concentration points NMAX [-] Results: Sensitivity to Meshing and Time step • Temperature 1400 °C

  12. Results: Isothermal Oxidation (As-received Zircaloy) • Temperature 1200 °C

  13. Results: Isothermal Oxidation (As-received Zry)Cont’d • Temperature 1300 °C

  14. Results: Transient Oxidation (As-received Zry) • Linear heat-up and cool-down between 800 and 1300 °C at 1 °C/s

  15. Oxygen Solubility in Hydrided Zircaloy • Absorbed hydrogen CHincreases the oxygen solubility SO in beta phase. • CEA 5, 6 experimental data available for 1200 °C. • Saturation of this effect at  600 wppm H. • Billone (ANL, 2005) 7: Fit to CEA data (additive term to Chung-Kassner3 correlation): SO = exp(5.02 – 8220 / T) + 0.6· (1 - exp[-0.006· CH]) [wt%] T[K], CH [ wppm]. • The increased solubility limitaccelerates the filling of betaphase with oxygen. 0.6 1200 °C

  16. Results: Isothermal Oxidation (Hydrided Zircaloy) • Beta layer poor in oxygen (~ < 0.6 wt%) disappears faster in hydrided Zircaloy. FBS = CAV / CB, FBSA = (CAV – CINI) / (CB - CINI), where CINI … initial oxygen conc. in as received Zry.

  17. Conclusions • Zry embrittlement module ZROB available since mid 2005 (ICARE2-V3mod1.4). • Applied embrittlement criteria: Chung-Kassner (1980) & Pawel (1974) … to be revised. • Effect of hydrogen is under testing: • Increased oxygen solubility due to H: ready for implementation into ZROB • Increased oxygen diffusion coefficient : • Impact of hydrides onto embrittlement : • References [ 1] H. M. Chung, T. F. Kassner: NUREG/CR-1344 (1980). [ 2] R. E. Pawel: Oxygen diffusion in beta Zircaloy during steam oxidation. J. Nucl. Mat. 50 (1974). [ 3] H. M. Chung, T. F. Kassner: Pseudobinary Zircaloy-Oxygen Phase Diagram. J. Nucl. Mat. 84 (1979) [ 4] H. S. Carslaw, J. C. Jaeger: Conduction of Heat in Solids. Oxford, Clarendon Press, 2nd edition (1959), p.100. [ 5] L. Portier et al.: 14th Int. Symp. Zirconium Nucl. Ind., June 13-17, 2004, Stockholm, to be published by ASTM. [ 6] J-C. Brachet et al.: NRC Nucl. Safety Research Conf., Oct. 25-28, 2004, Washington. [ 7] M. C. Billone: LOCA Embrittlement Criterion. Argonne National Laboratory (April 2005). … to be experimentally investigated

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